1. Field of the Invention
The present invention relates to a rate-responsive pacemaker of the type having at least one pulse generator which generates and emits stimulation pulses with a variable stimulation interval to a heart, a measurement device which generates a measurement signal corresponding to blood flow into or the volume of blood in a heart chamber, and a control device which controls the pulse generator's stimulation intervals depending on the measurement signal.
2. Description of the Prior Art
A known heart stimulator is described in U.S. Pat. No. 4,686,987 containing an impedance meter which measures heart impedance, a parameter which is proportional to ventricular volume and, accordingly, the volume of blood in the heart. The heart's stroke volume in each heart cycle is derived from the impedance signal, and the heart stimulator is controlled such that the stimulation rate causes the stroke volume to remain as constant as possible. In this known heart stimulator, any change in stroke volume thus indicates a change in the patient's level of physical activity and causes a change in the stimulation rate, thereby counteracting the change in stroke volume.
Another heart stimulator is described in U.S. Pat. No. 4,535,774 in which stroke volume is established either by measuring blood flow into the heart or by measuring impedance. The measured stroke volume is then used for setting a heartbeat rate. A relationship between stroke volume and heartbeat rate, in which stroke volume increases when the heartbeat rate increases, is utilized for optimizing cardiac function as much as possible, whereby the amount of blood pumped out of the heart every minute is sufficient for the body's needs without the heartbeat rate becoming excessively fast.
Both these known heart stimulators measure stroke volume and then utilize this parameter, or changes in same, for establishing an appropriate stimulation rate and then imposing that rate on the heart. The heart stimulator according to the former patent has the disadvantage that heartbeat rate rises rapidly when there is a need for a large volume of blood per unit of time (cardiac output). This problem is admittedly solved with a heart stimulator according to the latter patent in which stroke volume is allowed to change and increase when the heart rate increases, thereby retaining cardiac output without an excessively fast stimulation rate. In view of the normal course of events in a cardiac cycle, however, stroke volume is not particularly suitable for use in setting a heartbeat rate in this way. When blood in the heart has been expelled in a normal cardiac cycle and the muscle tissue relaxes for refilling, the influx of blood into the heart is governed by blood pressure in the vascular system. Blood flows rapidly into the heart at the beginning of diastole. The flow ultimately ceases when the heart is full of blood, i.e., when pressure in the heart and vascular system equalize. A slight change in stroke volume could therefore require a major change in rate. In addition, the influx of blood into the heart depends on the physical and mental condition of the person in whom the stimulation device is implanted. At rest, the influx of blood into a patient's heart is slower at the beginning of diastole than during physical exercise or stress.